For example, the unesterified copolymer is heated under reflux in an organic solvent (for example, a hydrocarbon such as benzene or toluene; a hologenated hydrocarbon such as chloroform, dichloromethane or chlorobenzene; or a ketone such as acetone, methyl ethyl ketone or cyclohexanone) or without a solvent, in the presence of a condensation catalyst (for example, a strong acid such as sulfuric ac